Date: 28 Jul 1995 00:00:01 EST
Allen Akin of SGI <akin@tuolumne.asd.sgi.com> says: It's not, for the
most common cases. After all, similar microcode and the same hardware
are used for both commands. However, there are three issues to keep
in mind.
First, some midrange and low-end SGI graphics adaptors (particularly
XS, XZ, Elan, and Extreme) transfer ABGR-ordered images much faster
than they transfer RGBA-ordered images. The normal image format in
IrisGL was ABGR, while in OpenGL it's RGBA. So to achieve the same
performance in OpenGL that you did in IrisGL on those machines, you
need to use ABGR-format images in OpenGL. The ABGR extension
available in Irix 5.3 and later releases allows you to do this. See
``man glintro'' for background information on using OpenGL extensions,
and ``man gldrawpixels'' for details on ABGR. Note that
RealityEngine, IMPACT, and all future machines will process RGBA data
at least as fast as ABGR, so RGBA is the way to go for new code.
Second, some OpenGL pixel data types are faster than others. For most
machines, unsigned byte RGBA (or ABGR) is the fastest full-color type.
Unsigned byte and unsigned short are usually the fastest gray-scale
types. Signed integer types are slower.
Third, OpenGL pixel operations have a much richer set of features than
IrisGL, and if any of those features are enabled, then image transfer
can be significantly slower. Always disable the features that you
don't need. The following code fragment disables features that are
likely to make glDrawPixels slow:
/*
* Disable stuff that's likely to slow down glDrawPixels.
* (Omit as much of this as possible, when you know in advance
* that the OpenGL state will already be set correctly.)
*/
glDisable(GL_ALPHA_TEST);
glDisable(GL_BLEND);
glDisable(GL_DEPTH_TEST);
glDisable(GL_DITHER);
glDisable(GL_FOG);
glDisable(GL_LIGHTING);
glDisable(GL_LOGIC_OP);
glDisable(GL_STENCIL_TEST);
glDisable(GL_TEXTURE_1D);
glDisable(GL_TEXTURE_2D);
glPixelTransferi(GL_MAP_COLOR, GL_FALSE);
glPixelTransferi(GL_RED_SCALE, 1);
glPixelTransferi(GL_RED_BIAS, 0);
glPixelTransferi(GL_GREEN_SCALE, 1);
glPixelTransferi(GL_GREEN_BIAS, 0);
glPixelTransferi(GL_BLUE_SCALE, 1);
glPixelTransferi(GL_BLUE_BIAS, 0);
glPixelTransferi(GL_ALPHA_SCALE, 1);
glPixelTransferi(GL_ALPHA_BIAS, 0);
/*
* Disable extensions that could slow down glDrawPixels.
* (Actually, you should check for the presence of the proper
* extension before making these calls. I've omitted that
* code for simplicity.)
*/
#ifdef GL_EXT_convolution
glDisable(GL_CONVOLUTION_1D_EXT);
glDisable(GL_CONVOLUTION_2D_EXT);
glDisable(GL_SEPARABLE_2D_EXT);
#endif
#ifdef GL_EXT_histogram
glDisable(GL_HISTOGRAM_EXT);
glDisable(GL_MINMAX_EXT);
#endif
#ifdef GL_EXT_texture3D
glDisable(GL_TEXTURE_3D_EXT);
#endif

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